Optical fiber for extended wavelength band

ABSTRACT

An optical transmission fiber for use in a wavelength division multiplexing transmission system is disclosed. The transmission fiber includes an inner core surrounded by a first, second and at least a third glass layer along the length of the fiber. The first glass layer has a depressed refractive-index difference and the second glass layer has a refractive-index difference of substantially zero. The third glass layer has a positive refractive-index difference. The fiber has an improved relationship between dispersion slope and depressed profile volume. The fiber can have a dispersion value of at least 1.5 ps/nm/km and a dispersion slope of less than about 0.07 ps/nm 2 /km over an extended range of carrier wavelengths for the transmission system, such as the range 1450-1650 nm.

BACKGROUND OF THE INVENTION

[0001] The present invention relates generally to an opticaltransmission fiber that has improved dispersion characteristics acrossthe low attenuation band of optical fibers, and specifically to anoptical transmission fiber for use in a wavelength-division-multiplexingtransmission system that has low attenuation and tailored dispersioncharacteristics across the bandwidth of 1450-1650 nm.

[0002] In optical communication systems, non-linear optical effects areknown to degrade the quality of transmission along standard transmissionoptical fiber in certain circumstances. These non-linear effects, whichinclude four-wave mixing, self-phase modulation, Brillouin scattering,Raman scattering, and cross-phase modulation, induce distortion into thetransmitted signal in high-power systems, thereby degrading the qualityof the transmission. In particular, the non-linear effects can hamperquality transmission using wavelength division multiplexing (WDM), whichotherwise greatly enhances the signal carrying capability of opticaltransmission fibers by increasing the number of transmission channelsthrough which signals may be sent.

[0003] These non-linear effects can be minimized or avoided by usingsingle-mode transmission fibers that have a large effective area. Inaddition, the phenomenon of four-wave mixing can be minimized by fibershaving an absolute value of dispersion that is greater than zero at oraround the operating wavelengths. However, in advanced WDM systems, suchas Dense Wavelength Division Multiplexing (DWDM) and Hyper-DenseWavelength Division Multiplexing (HDWDM) systems, where the transmissionchannels are closely packed together (spacing ≦0.4 nm), the value ofdispersion must meet a minimum value to maintain the quality of thesignals. On the other hand, if the dispersion value of the fiber becomestoo large, the signals will become distorted during transmission unlessdispersion correction devices are included in the transmission line.Thus, for an optical fiber to be effective in a WDM system, the fibermust have a minimum dispersion, but the value of dispersion must also bebelow a maximum value.

[0004] Optical fibers in general exhibit low attenuation across awavelength range of about 1450-1650 nm. Indeed, the minimum spectralattenuation for standard optical fibers occurs at around 1580 nm, whileintrinsic fiber attenuation remains typically below 0.27 dB/km fordispersion-shifted fibers up to around 1650 nm and even lower fordispersion unshifted fibers. However, conventional optical-fiberamplifiers doped with rare-earth materials such as erbium operate mosteffectively in a more limited wavelength window between around 1530-1565nm. As a result, some research has focused on minimizing non-lineareffects and attenuation for WDM systems across the wavelength range of1530-1565 nm.

[0005] Due to recent technological advances in optical amplifiers, thetransmission window of operating wavelengths for WDM systems isincreasing from the traditional wavelength range of 1530-1565 nm to amuch broader wavelength range of around 1450-1650 nm. Some publicationshave discussed working at lower wavelength regions down to 1470 nm. Inthis regard, Electronics Letters, vol. 34, no. 11, pp. 1118-1119 (May28, 1998) discusses an eight-channel WDM system operating from 1467 nmto 1478 nm, based on Thulium-doped fiber amplifiers. Others haveaddressed extending the operating bandwidth toward higher wavelengthregions up to about 1600 nm. See, e.g., Srivastava et al. ‘1 Tb/sTransmission of 100 WDM 10 Gb/s Channels Over 400 km of TrueWave™ Fiber’PD10, OFC'98. See also M. Jinno et al. ‘First demonstration of 1580 nmwavelength band WDM transmission for doubling usable bandwidth andsuppressing FWM in DSF’ Electronics Letters, vol. 33 no. 10 pp. 882-883(May 8, 1997). This extended range of available operating wavelengths isdue to a use of gain-shifted erbium-doped amplifiers.

[0006] In addition, the trend of expanding the amplification window issupported by the low attenuation of transmission fibers over theexpanded transmission window between 1450 and 1650 nm. However, existingfibers are severely limited in their transmission capabilities outsideof the traditional transmission window around 1550 nm. For example,currently available Non-Zero Negative Dispersion (NZD−) fibers have azero-dispersion wavelength λ₀ at approximately 1585 nm and are thereforenot suited for WDM transmissions because of non-linear effects at thisoperating wavelength. Similarly, Non-Zero Positive Dispersion (NZD+) andLarge Effective Area (LEA) fibers have zero-dispersion wavelengths λ₀ inthe area of 1500 nm and are therefore not suited for WDM transmissionsat this operating wavelength. Thus, because of the associated non-lineareffects, conventional fibers are not capable of supporting the newlybroadened transmission window. Moreover, for NZD+ and LEA fibers, evenif the transmission wavelengths were restricted to the band above 1530nm, the dispersion at around 1600 nm and at higher wavelengths would behigh, due to the steep slope of the dispersion curve, thus requiringdispersion compensation. Accordingly, Applicant has identified a needfor an optical transmission fiber that is capable of supporting WDMtransmissions across the transmission window from 1450 nm to 1650 nmthat provides suitable dispersion characteristics, low attenuation, andresistance to non-linear effects.

[0007] Various patents and publications have discussed optical fibersfor high performance communication systems. For example, U.S. Pat. No.5,553,185 to Antos et al., discloses a NZD fiber that is characterizedby a series of core regions each having a refractive-index profile andradius. The shape of the refractive-index profiles, in terms of therefractive-index difference and the radius, of each region may beadjusted to have properties tailored for a high performancetelecommunication system. In particular, one of the regions has adepressed refractive-index difference. The dispersion slope of thedisclosed fiber is less than 0.05 ps/nm²/km and the absolute value ofthe total dispersion is between 0.5 and 3.5 ps/nm/km over a pre-selectedtransmission range.

[0008] Another fiber for a high performance communication system isdiscussed in Y. Akasaka et al., Enlargement of Effective Core Area onDispersion-Flattened Fiber and Its Low Non-Linearity, OFC '98 TechnicalDigest, pp. 302-304. This fiber is also characterized by a series ofcore regions having varying refractive-index differences and radii. Oneof the core regions also has a depressed refractive-index difference.The disclosed fiber has a low dispersion slope over the transmissionwindow.

[0009] Lucent Technologies provided a press release in June 1998introducing its TrueWave® RS Fiber that has a reduced slope ofdispersion. According to the release, the new fiber has a dispersionslope across a wavelength band of about 1530-1620 nm with a low value,such that the dispersion ranges from about 3.5-7.5 ps/nm-km. The pressrelease does not disclose the refractive index profile of the TrueWave®RS Fiber.

[0010] U.S. Pat. No. 4,852,968 discloses a single mode optical fiberwhose refractive index profile comprises a depressed-index or trenchregion in the cladding region. By suitable adjustment of the position,width and index of the trench region, one or more fiber characteristicscan be improved, relative to a similar fiber that does not comprise anindex trench, such as: the slope of the chromatic dispersion curve atthe zero dispersion wavelength; the spectral value over which theabsolute value of the chromatic dispersion is less than a predeterminedvalue; the maximum absolute value of the chromatic dispersion in a givenspectral range; the bending loss at a given bend radius; the ratioa_(d)/a₁; the optical quality of the tube-derived material; theintegrated mode power at a_(d); the dopant concentration in the-core;and the dependence of λ₀ on the core radius.

[0011] U.S. Pat. No. 5,781,684 discloses a single mode optical waveguidehaving large effective area, achieved by using a segmented core profilewhich includes at least a segment, or a part of one segment, having arefractive index less than the minimum refractive index of the cladlayer. Dispersion slope values above 0.085 ps/nm²/km are disclosed.

[0012] U.S. Pat. No. 5,684,909 discloses a single mode optical waveguidehaving a core refractive index profile of at least four segments. Themain features of the core design are: at least two non-adjacent coreprofile segments have positive Δ%; and at least two non-adjacentsegments have negative Δ%. The waveguide structure lends itself to themanufacture of dispersion managed waveguide fibers.

[0013] Throughout the present description reference is made torefractive index profiles of optical fibers. The refractive indexprofiles comprise various radially arranged sections. Reference is madein the present description to precise geometrical shapes for thesesections, such as step, alpha-profile, parabola. It is evident that therefractive index profiles achieved in practice may differ from theabove, idealized, profiles. It has been shown in the literature,however, that these differences do not change the fiber characteristicsif they are kept under control. See, for example, U.S. Pat. No.4,406,518 (Hitachi).

[0014] In general, a refractive index profile has an associatedeffective refractive index profile which is different in shape. Aneffective refractive index profile may be substituted, for itsassociated refractive index profile without altering the waveguideperformance. For example, see “Single Mode Fiber Optics”, Luc B.Jeunhomme, Marcel Dekker Inc., 1990, page 32, section 1.3.2.

[0015] It will be understood that disclosing and claiming a particularrefractive index profile shape, includes the associated equivalents, inthe disclosure and claims.

SUMMARY OF THE INVENTION

[0016] Applicant has discovered that transmission fibers that operateover an extended operating range with suitable dispersion values buthave a region within the fiber core with exclusively a depressedrefractive index are subject to several disadvantages. In particular,these fibers have a high attenuation, which is due, in part, to theregion of depressed refractive index. In addition, the disclosedprofiles that have an annular core region of exclusively a depressedrefractive index between a central raised-index region and araised-index ring often result in diffusion of dopants between layersduring the manufacturing process, which degrades the quality of therefractive-index profile. In general, an optical transmission fiberconsistent with the present invention involves a single-mode opticaltransmission fiber for use in a wavelength division multiplexing systemthat has carrier wavelengths ranging from 1450 nm to 1650 nm. The fiberhas a glass core that includes an inner core having a firstrefractive-index difference, a first layer radially surrounding theinner core along the length of the fiber and having a secondrefractive-index difference of less than zero, a second layer radiallysurrounding the first layer along the length of the fiber and having athird refractive-index difference of substantially zero, a third layerradially surrounding the second layer along the length of the fiber andhaving a fourth refractive-index difference of greater than zero, and afourth layer radially surrounding the third layer along the length ofthe fiber and having a fifth refractive-index difference of less thanzero. A glass cladding surrounds the glass core and has arefractive-index difference substantially equal to zero. In addition,the slope of the dispersion curve is less than about 0.07 ps/nm²/km (andpreferably less than about 0.05 ps/nm²/km) over the range of carrierwavelengths.

[0017] In another aspect, a fiber consistent with the present inventioninvolves a single-mode optical transmission fiber for use in awavelength division multiplexing system that has carrier wavelengthsranging from 1450 nm to 1650 nm. The fiber has a glass core thatincludes an inner core having a first refractive-index difference, afirst layer radially surrounding the inner core along the length of thefiber and having a second refractive-index difference of less than zero,a second layer radially surrounding the first layer along the length ofthe fiber and having a third refractive-index difference ofsubstantially zero, a third layer radially surrounding the second layeralong the length of the fiber and having a fourth refractive-indexdifference of greater than zero. A glass cladding surrounds the glasscore and has a refractive-index difference substantially equal to zero.In addition, the slope of the dispersion curve is less than about 0.07ps/nm²/km (and preferably less than about 0.05 ps/nm²/km) over the rangeof carrier wavelengths.

[0018] In another aspect, a fiber consistent with the present inventioninvolves a single-mode optical transmission fiber for use in awavelength division multiplexing transmission system having carrierwavelengths ranging between about 1530 nm and 1650 nm. The fiber of thisembodiment has a zero-dispersion wavelength of about 1480 nm.

[0019] In another aspect, the invention includes a single-mode opticaltransmission fiber that has a glass core having a centralcross-sectional area with a first refractive-index peak, an outside ringwith a second refractive-index peak, a first intermediate region betweenthe two peaks having a low-dopant content, and a second intermediateregion between the first peak and the first intermediate region, with arefractive-index depression lower than the first intermediate region.The fiber may also have a layer radially surrounding the secondrefractive-index peak and having a depressed refractive-indexdifference. The fiber also has a glass cladding surrounding the glasscore, wherein the fiber has a dispersion value of at least 1.5 ps/nm/kmand a dispersion slope of less than about 0.07 ps/nm²/km (and preferablyless than about 0.05 ps/nm²/km) over a wavelength range of about1450-1650 nm.

[0020] In a further aspect the invention includes a method for producinga single-mode optical fiber for use in awavelength-division-multiplexing transmission system having carrierwavelengths ranging between about 1450-1650 nm, comprising the steps of:producing a preform having an inner core region with a firstrefractive-index difference; a first layer radially surrounding theinner core region along the length of the preform and having a secondrefractive-index difference of less than zero, a second layer radiallysurrounding the first layer along the length of the preform and having athird refractive-index difference that, in absolute value, is less than40% of said second refractive-index difference, a third layer radiallysurrounding the second layer along the length of the preform and havinga fourth refractive-index difference of greater than zero; and a glasscladding layer surrounding the core region and having a refractive-indexdifference substantially equal to zero; and drawing said preform.

[0021] It is to be understood that both the foregoing generaldescription and the following detailed description are exemplary andexplanatory only, and are not restrictive of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] The accompanying drawings, which are incorporated and constitutea part of this specification, illustrate several embodiments of theinvention, and together with the description, serve to explain theprinciples of the invention.

[0023]FIG. 1 is a cross section of an optical transmission fiberconsistent with an embodiment of the present invention;

[0024]FIG. 2 is a cross section of an optical transmission fiberconsistent with another embodiment of the present invention;

[0025]FIG. 3 is a graph illustrating an exemplary refractive-indexprofile of a fiber according to the present invention;

[0026]FIG. 4 is a graph illustrating another exemplary refractive-indexprofile of a fiber according to the present invention;

[0027]FIG. 5 is a graph illustrating the refractive-index profile of aconventional low dispersion slope fiber;

[0028]FIG. 6 is a graph illustrating the chromatic dispersion value as afunction of transmission wavelength for two alternative embodiments of afiber of the present invention and also illustrating the attenuationvalue as a function of transmission wavelength of a fiber according tothe present invention;

[0029]FIG. 7 is a graph illustrating a first example of a fiberaccording to the present invention;

[0030]FIG. 8 is a graph illustrating a second example of a fiberaccording to the present invention;

[0031]FIG. 9 is a graph illustrating a third example of a fiberaccording to the present invention;

[0032]FIG. 10 is a graph illustrating a fourth example of a fiberaccording to the present invention;

[0033]FIG. 11 is a graph illustrating a fifth example of a fiberaccording to the present invention;

[0034]FIG. 12 is a graph illustrating the relation between dispersionslope and depressed profile volume and for a set of fibers havingrefractive index profiles according to the conventional design of FIG.5;

[0035]FIG. 13 is a graph illustrating the relation between dispersionslope and depressed profile volume and for a set of fibers havingrefractive index profiles according to the invention, and

[0036]FIG. 14 is a refractive index profile of a fiber preform producedby Applicant by the MCVD method.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0037] Reference will now be made in detail to the present preferredembodiments of the invention, examples of which are illustrated in theaccompanying drawings. Wherever possible, the same reference numberswill be used throughout the drawings to refer to the same or like parts.

[0038] Optical fibers consistent with the present invention have arefractive-index profile that includes an area of depressedrefractive-index difference adjacent an area having a refractive-indexdifference of substantially zero. Applicant has discovered that opticaltransmission fibers having refractive-index profiles of this nature canproduce optical transmission characteristics in an operating wavelengthrange of between about 1450 nm and 1650 nm that include a moderatedispersion value at the lowest operating wavelength, a low dispersionslope, and a low attenuation.

[0039] Applicant has further discovered that optical transmission fibersincluding this refractive-index profile can effectively supportWavelength Division Multiplexing (WDM) and Hyper-Dense WDM opticaltransmissions over the operating wavelength transmission window between1450 nm and 1650 nm.

[0040] As shown in FIG. 5, conventional optical fibers that have anannular region of depressed refractive index include an inner core 52having a first refractive-index difference Δn₁. A first glass layer 54may surround inner core 52 as the annular region and have a depressedrefractive index difference Δn₂. A second glass layer 56 may surroundthe first glass layer and have a peak refractive index difference Δn₃within its width that is less than the peak refractive index withininner core 52 but greater than zero. A cladding layer 58 surrounds thesecond glass layer and has a refractive-index difference ofsubstantially zero. In an alternative embodiment, an outer glass layer59 with a negative refractive-index difference is arranged outsidesecond glass layer 56, as shown by the dashed lines. Outer layer 59 maydirectly surround second glass layer 56.

[0041] The profile of FIG. 5 can be characterized by the volume of thedepression that extends across annular layer 54. If r1 denotes the outerradius of the inner core 52 and r3 denotes the inner radius of secondglass layer 56, then the fiber depressed profile volume is given by thefollowing formula: $\begin{matrix}{\int_{r_{1}}^{r_{3}}{\Delta \quad {n \cdot r}{\quad r}}} & (1)\end{matrix}$

[0042] Applicant has found that optical transmission fibers havingrefractive-index profiles according to the present invention are bettersuited for use with WDM transmissions that range from about 1450 nm to1650 nm than conventional fibers such as the fiber of FIG. 5. Inparticular, Applicant has discovered that by including in thecross-sectional outer part of the annular depression, a second glasslayer having a refractive-index difference of substantially less, inabsolute value, than the depressed refractive index difference, thedepressed profile volume of the fiber according to Equation (1) above isreduced when compared to prior art fibers, while a desirably lowdispersion slope can be achieved across the transmission window. Thelower depressed profile volume results in lower amount of negativedopant and thus ease of manufacture of the fiber and lower attenuation.In addition, the fibers have a moderate value of dispersion at the lowerend of the transmission window.

[0043] An optical fiber according to a preferred embodiment of thepresent invention is schematically illustrated in FIG. 1 and isgenerally designated by the reference number 10. In accordance with thepresent invention, an optical transmission fiber for use in a WDMtransmission system includes a glass core with an inner core having afirst refractive-index difference Δn₁, a first layer radiallysurrounding the inner core along the length of the fiber and having asecond refractive-index difference Δn₂ of less than zero, a second layerradially surrounding the first layer along the length of the fiber andhaving a third refractive-index difference Δn₃ of substantially zero, athird layer radially surrounding the second layer along the length ofthe fiber and having a fourth refractive-index difference Δn₄ of greaterthan zero. A glass cladding surrounds the glass core and has arefractive-index difference substantially equal to zero. Preferably thecore comprises a fourth layer radially surrounding the third layer alongthe length of the fiber and having a fifth refractive-index differenceΔn₅ of less than zero.

[0044] The fiber has a dispersion value of at least 1.5 ps/nm/km overthe carrier wavelength range (preferably over 2.5 ps/nm/km for dense WDMtransmission) and a dispersion slope less than about 0.07 ps/nm²/km(preferably less than about 0.05 ps/nm²/km) over the carrier wavelengthrange.

[0045] As schematically illustrated in FIG. 1 (not-to-scale), opticalfiber 10 includes a plurality of light conducting layers of glass. Theaxial center of fiber 10 is inner core 12, which is made of doped glass.Inner core 12 has a first refractive-index difference Δn₁ and a radiusr₁. The refractive-index difference refers to the difference inrefractive index between a given layer of glass and the cladding glass.That is, for example, the refractive-index difference Δn₁ of inner core12 equals n₁−n_(cladding). Δn₁ can be chosen in the range 0.004-0.010,while r₁ can be chosen in the range 2-5 μm. Preferred ranges for Δn₁ andfor r₁ are respectively 0.005-0.008 and 3-4 μm. Preferably, inner core12 is made of SiO₂ doped with a substance that increases the refractiveindex of pure SiO₂ such as GeO₂.

[0046] A first glass layer 14 radially surrounds inner core 12 along thelength of fiber 10. First glass layer 14 extends from the outer radiusr₁ of the inner core to a radius r₂ and has a depressed index ofrefraction Δn₂ across its width. As is well known in the art, adepressed index of refraction exists when the index of refraction of agiven glass layer is less than the refractive index of the claddinglayer, i.e. Δn₂, as given by the above equation, is less than 0. As alsoknown in the art, the dispersion slope of a fiber may, in general, bereduced by including a layer of glass having an area of depressedrefractive index. Δn₂ can be chosen in the range −0.006-0.001, while apreferred range for Δn₂ is −0.003 to −0.002. Preferably, first glasslayer 14 is made of SiO₂ doped with a substance that decreases therefractive index of pure SiO₂, such as fluorine. The width of firstglass layer 14 can be chosen in the range 1-6 μm, a preferred rangebeing 2-4 μm.

[0047] A second glass layer 16 radially surrounds first glass layer 14along the length of fiber 10. Second glass layer 16 extends from theouter radius r₂ of the first glass layer to a radius r₃ and has an indexof refraction Δn₃ within its width. The index of refraction Δn₃ ofsecond glass layer 16 is, in absolute value, less than about 40% of Δn₂,preferably less than about 20% of Δn₂. Preferably, second glass layer ismade of SiO₂, although the second glass layer may be made of anymaterial or combination of materials having a refractive-indexdifference substantially equal to the refractive index of the claddinglayer, described below. The width of second glass layer 16 can be chosenin the range 1-5 μm, a preferred range being 2-4 μm.

[0048] A third glass layer 18 radially surrounds second glass layer 16along the length of fiber 10. Third glass layer extends from the outerradius r₃ of second glass layer 16 to an outer radius r₄. The thirdglass layer has a maximum refractive index Δn₄. Δn₄ can be chosen in therange 0.003-0.010, while a preferred range for Δn₄ is 0.004-0.008. Thewidth of third glass layer 18 can be chosen in the range 1-4 μm, apreferred range being 2-3 μm.

[0049] A fourth glass layer 15 radially surrounds third glass layer 18along the length of fiber 10. Fourth glass layer extends from the outerradius r₄ of third glass layer 18 to an outer radius r₅. The fourthglass layer has a refractive index difference Δn₅ that is less thanzero. Δn₅ can be chosen in the range −0.003 to 0.0, while a preferredrange for Δn₅ is −0.002 to 0.0. The width of fourth glass layer 15 canbe chosen in the range 1-6 μm, a preferred range being 3-5 μm.

[0050] Finally, a light conducting cladding 19 surrounds the third glasslayer 18 in a conventional manner to help guide light propagating alongthe axis of fiber 10. Cladding 19 may comprise pure SiO₂ glass with arefractive-index difference substantially equal to zero or include arefractive-index modifying dopant.

[0051] In an alternative embodiment, showed by dashed line, a fifthglass layer 17 having a refractive index difference Δn₆ that issubstantially zero is comprised between third glass layer 18 and fourthglass layer 15.

[0052] A particular embodiment, as shown in FIG. 2, derives from theembodiments of FIG. 1 when cladding 19 directly surrounds third glasslayer 18, so that fourth and fifth glass layers are absent. However, theembodiments of FIG. 1, having an outer depressed glass layer 15 with orwithout a fifth glass layer 17, are preferred because they may simplifythe achievement of single mode behavior at the operation wavelength.

[0053]FIG. 3 illustrates a refractive-index profile 20 across the radiusof fiber 10 for a first embodiment of the present invention, where axis32 indicates the axial center of fiber 10 and axis 34 denotes arefractive-index difference of substantially zero. As shown,refractive-index profile has a first layer 24 having a depressedrefractive index Δn₂ followed by a second layer 26 having arefractive-index difference of substantially zero. Layers 24 and 26provide a depressed trench between inner core layer 22 and outer peak28. In one preferred embodiment, the refractive-index difference Δn₄ ofouter peak 28 is less than the refractive-index difference Δn₁ of innercore 22. In an alternative embodiment, as indicated by reference number23, the refractive-index difference Δn₄ of outer peak 28 may be greaterthan the refractive-index difference Δn₁ of inner core 22. As shown, anouter layer 29 having a depressed refractive-index difference is placedoutside outer peak 28. Outer layer 29 may surround outer peak 28 or, inan alternative embodiment, an intermediate layer 27 having arefractive-index difference of substantially zero may be arrangedbetween outer peak 28 and outer layer 29.

[0054]FIG. 4 shows another refractive-index-profile, corresponding tothe embodiment of FIG. 2, wherein cladding 30 directly surrounds outerpeak 28. The refractive-index profile of FIG. 4 differs from that ofFIG. 3 in that outer layer 29 and intermediate layer 27 are absent.

[0055]FIG. 6 illustrates exemplary dispersion curves 40 and 42 andattenuation curve 44 a fiber 10 having a refractive-index profileaccording to the present invention. As shown in curve 44, theattenuation of fiber 10 reaches a peak 46 at a wavelength smaller thanthe transmission window of 1450-1650 nm. The attenuation of fiber 10 isless than about 0.27 dB/km over the transmission window from about 1450nm to 1650 nm.

[0056] As also shown in 40, the dispersion at 1450 nm of a fiber 10according to a first embodiment is about 1.5 ps/nm/km. The slope of thedispersion curve is less than 0.06 ps/nm²/km across the bandwidth of1450 nm to 1650 nm. Fiber 10 has a dispersion value less than about 12ps/nm/km at 1650 nm. Curve 42 depicts a second embodiment of the presentinvention where the zero-dispersion wavelength occurs at around 1480 nm.A fiber 10 according to this second embodiment has a dispersion value ofless than about 9 ps/nm/km at 1650 nm.

[0057] In addition, the effective area of fiber 10 at 1550 nm is greaterthan about 50 μm². As is readily known in the art, a large effectivearea will help limit the impact of non-linear effects. However, thedispersion slope of the fiber increases as the effective area of thefiber increases. The fiber of the present invention has an effectivearea that is large enough to limit the impact of non-linear effectswhile being small enough to achieve an optimal trade off betweeneffective area and dispersion slope.

[0058] The transmission characteristics of a first embodiment of a fiber10 having a refractive-index profile according to the present inventionare provided in Table 1. TABLE 1 Optical Transmission CharacteristicsCable cut-off wavelength <1400 nm Effective Area ≧50 μm² Dispersion at1450 nm D ≧ 1.5 ps/nm/km (preferably ≧ 2.5 ps/nm/km) Dispersion Slope @1550 nm ≦0.070 ps/nm²/km (preferably ≦ 0.050 ps/nm²/km) Attenuation @1310 nm ≦0.45 dB/km Attenuation @ 1550 nm ≦0.30 dB/km

[0059] FIGS. 7-9 illustrate specific examples of fibers havingrefractive-index profiles according to a first embodiment of the presentinvention.

EXAMPLE 1

[0060] As shown in FIG. 7, inner core 72 has a substantially constantrefractive-index difference Δn₁ of about 0.0063 and extends for a radiusr₁ of about 3.3 μm. The refractive-index difference of inner core 72 maybe increased by doping the width of the inner core with GeO₂ or anyother well-known refractive-index-increasing dopant. Although FIG. 7depicts inner core 72 as having sharp edges, its profile may be roundedin actual implementation.

[0061] First glass layer 74 has a depressed refractive-index differenceΔn₂ of about −0.0010 and extends for a radial distance of about 2.8 μm.Depressed profile volume is about −0.013 μm². The refractive-indexdifference of first glass layer 74 may be decreased by doping the widthof the first core layer with fluorine or any other well-knownrefractive-index-decreasing dopant. Second glass layer 76 has arefractive-index difference Δn₃ of about 0 and extends for a radialdistance of about 2.0 μm.

[0062] Third glass layer 78 has a substantially parabolic profile andreaches a maximum refractive index Δn₄ of about 0.0032 at a midpointwithin its width of about 2.9 μm. The refractive-index difference of thethird glass layer may be formed by doping the glass layer withincreasing amounts of GeO₂, or other well-knownrefractive-index-increasing dopant.

[0063] Third glass layer 78 is surrounded by cladding 80 that has arefractive-index difference of about 0.

[0064] The specific embodiment of fiber 10 illustrated in FIG. 7 has thefollowing optical transmission characteristics, which have beengenerated through computer simulations:

[0065] Cable cut off ≦1400 nm

[0066] Dispersion at 1450 nm=1.9 ps/nm/km

[0067] Dispersion Slope at 1450 nm=0.050 ps/nm²/km

[0068] Dispersion at 1550 nm=6.5 ps/nm/km

[0069] Dispersion Slope at 1550 nm=0.046 ps/nm²/km

[0070] Mode Field Diameter at 1550 nm=9.1 μm

[0071] Effective Area at 1550 nm=63 μm²

[0072] Non Linearity Coefficient γ=1.4 W⁻¹ km⁻¹

[0073] Macrobending attenuation <0.5 dB for 100 turns on a 60 mmdiameter mandrel

[0074] Microbending sensitivity=2.9 (dB/km)/(g/mm) as determined by theexpandable bobbin method, as described for example in G. Grasso and F.Meli “Microbending losses of cabled single-mode fibers”, ECOC '88, page526-ff., or in G. Grasso et al. “Microbending effects in single-modeoptical cables”, International Wire and Cable Symposium, 1988, page722-ff.

EXAMPLE 2

[0075] In another embodiment and as shown in FIG. 8, inner core 72 has asubstantially constant refractive-index difference Δn₁ of about 0.0067and extends for a radius r₁ of about 3.2 μm. The refractive-indexdifference of inner core 72 may be increased by doping the width of theinner core with GeO₂ or any other well-known refractive-index-increasingdopant.

[0076] First glass layer 74 has a depressed refractive-index differenceΔn₂ of about −0.0015 and extends for a radial distance of about 3.4 μm.Depressed profile volume is about −0.025 μm². The refractive-indexdifference of first glass layer 74 may be decreased by doping the widthof the first core layer with fluorine or any other well-knownrefractive-index-decreasing dopant. Second glass layer 76 has arefractive-index difference Δn₃ of about 0 and extends for a radialdistance of about 2.2 μm.

[0077] Third glass layer 78 has a substantially parabolic profile andreaches a maximum refractive index Δn₄ of about 0.0090 at a midpointwithin its width of about 1.1 μm. The refractive-index difference of thethird glass layer may be formed by doping the glass layer withincreasing amounts of GeO₂, or any other well-knownrefractive-index-increasing dopant.

[0078] As can be seen, this embodiment of FIG. 8 has its outer peak 78having refractive-index difference higher than inner peak 72. Thirdglass layer 78 is surrounded by cladding 80 that has a refractive-indexdifference of about 0.

[0079] The specific embodiment of fiber 10 illustrated in FIG. 8 has thefollowing optical transmission characteristics.

[0080] Cable cut off ≦1400 nm

[0081] Dispersion at 1450 nm=1.6 ps/nm/km

[0082] Dispersion Slope at 1450 nm=0.042 ps/nm²/km

[0083] Dispersion at 1550 nm 5.0 ps/nm/km

[0084] Dispersion Slope at 1550 nm 0.036 ps/nm²/km

[0085] Mode Field Diameter at 1550 nm=8.6 μm

[0086] Effective Area at 1550 nm=57 μm²

[0087] Non Linearity Coefficient γ=1.6 W⁻¹ km⁻¹

[0088] Macrobending attenuation <0.5 dB for 100 turns on a 60 mmdiameter mandrel

[0089] Microbending sensitivity=2.1 (dB/km)/(g/mm) as determined by theexpandable bobbin method.

EXAMPLE 3

[0090] In another preferred embodiment and as shown in FIG. 9, innercore 72 of fiber 10 has a substantially constant refractive-indexdifference Δn₁ of about 0.0067 and extends for a radius r₁ of about 3.15μm. The refractive-index difference of inner core 72 may be increased bydoping the width of the inner core with GeO₂ or any other well-knownrefractive-index-increasing dopant.

[0091] First glass layer 74 has a depressed refractive-index differenceΔn₂ of about −0.0014 and extends for a radial distance of about 3.1 μm.Depressed profile volume is about −0.021 1 μm². The refractive-indexdifference of first glass layer 74 may be decreased by doping the widthof the first core layer with fluorine or any other well-knownrefractive-index-decreasing dopant. Second glass layer 76 has arefractive-index difference Δn₃ of about 0 and extends for a radialdistance of about 3.0 μm Third glass layer 78 has a substantiallyparabolic profile and reaches a maximum refractive index Δn₄ of about0.0040 at a midpoint within its width of about 3.2 μm. Therefractive-index difference of the third glass layer may be formed bydoping the glass layer with increasing amounts of GeO₂, or any otherwell-known refractive-index-increasing dopant.

[0092] Third glass layer 78 is surrounded by a fourth glass layer 79that has a refractive-index difference of about −0.0011 along its widthof 4.0 μm.

[0093] Fourth glass layer 79 is surrounded by cladding 80 that has arefractive-index difference of about 0.

[0094] The specific embodiment of fiber 10 illustrated in FIG. 9 has thefollowing optical transmission characteristics.

[0095] Cable cut off ≦1400 nm

[0096] Dispersion at 1450 nm=1.6 ps/nm/km

[0097] Dispersion Slope at 1450 nm=0.038 ps/nm²/km

[0098] Dispersion at 1550 nm=5.1 ps/nm/km

[0099] Dispersion Slope at 1550 nm=0.034 ps/nm²/km

[0100] Mode Field Diameter at 1550 nm=8.6 μm

[0101] Effective Area at 1550 nm=56 μm²

[0102] Non Linearity Coefficient γ=1.6 W⁻¹ km⁻¹

[0103] Macrobending attenuation <0.5 dB for 100 turns on a 60 mmdiameter mandrel

[0104] Microbending sensitivity=2.0 (dB/km)/(g/mm) as determined by theexpandable bobbin method.

[0105] In accordance with the present invention, the opticaltransmission fiber having a refractive-index profile as described hereinmay be used with a WDM transmission system that operates at largerwavelengths. In particular, a second embodiment of the invention fibermay be used with WDM transmission systems that have carrier wavelengthsin the range of about 1530 nm to 1650 nm. FIG. 4 illustrates adispersion curve 42 for a fiber according to this second embodiment.

[0106] As shown in FIG. 4, the zero-dispersion wavelength of fiber 10 isshifted to around 1480 nm. The dispersion slope of dispersion curve 42is preferably less than about 0.06 ps/nm²/km. The resulting dispersionvalue over the larger wavelengths is thereby reduced. Preferably, fiber10 has a dispersion less than about 9 ps/nm/km at the largertransmission wavelength of 1650 nm. The optical transmissioncharacteristics of the fiber of this second embodiment are presented inTable 2. TABLE 2 Optical Transmission Characteristics Cable cut-offwavelength (λ_(cc)) <1500 nm Effective Area ≧50 μm² Dispersion at 1530nm D ≧ 1.5 ps/nm/km (preferably ≧ 2.5 ps/nm/km) Dispersion Slope @ 1550nm ≦0.070 ps/nm²/km (preferably ≦ 0.050 ps/nm²/km) Attenuation @ 1310 nm≦0.45 dB/km Attenuation @ 1550 nm ≦0.30 dB/km

[0107] FIGS. 10-11 illustrate specific examples of fibers havingrefractive-index profiles according to a second embodiment of thepresent invention.

EXAMPLE 4

[0108]FIG. 10 illustrates a refractive-index profile of a fiberaccording to this second embodiment. Inner core 72 of fiber 10 has asubstantially constant refractive-index difference Δn₁ of about 0.0066and extends for a radius r₁ of about 3.2 μm. The refractive-indexdifference of inner core 72 may be increased by doping the width of theinner core with GeO₂ or any other well-known refractive-index-increasingdopant.

[0109] First glass layer 74 has a depressed refractive-index differenceΔn₂ of about −0.0013 and extends for a radial distance of about 3.3 μm.Depressed profile volume is about −0.021 μm². The refractive-indexdifference of first glass layer 74 may be decreased by doping the widthof the first core layer with fluorine or any other well-knownrefractive-index-decreasing dopant. Second glass layer 76 has arefractive-index difference Δn₃ of about 0 and extends for a radialdistance of about 2.4 μm.

[0110] Third glass layer 78 has a substantially parabolic profile andreaches a maximum refractive index Δn₄ of about 0.0058 at a midpointwithin its width of about 2.1 μm. The refractive-index difference of thethird glass layer may be formed by doping the glass layer withincreasing amounts of GeO2, or any other well-knownrefractive-index-increasing dopant.

[0111] Third glass layer 78 is surrounded by a fourth glass layer 79that has a refractive-index difference of about −0.0008 along the widthof 4.3 μm.

[0112] Fourth glass layer 79 is surrounded by cladding 80 that has arefractive-index difference of about 0.

[0113] The specific embodiment of fiber 10 illustrated in FIG. 10 hasthe following optical transmission characteristics.

[0114] Cable cut off ≦1500 nm

[0115] Dispersion at 1550 nm=3.3 ps/nm/km

[0116] Dispersion Slope at 1550 nm=0.038 ps/nm²/km

[0117] Mode Field Diameter at 1550 nm=8.7 μm

[0118] Effective Area at 1550 nm=59 μm²

[0119] Non Linearity Coefficient γ=1.5 W⁻¹ km⁻¹

[0120] Macrobending attenuation <0.5 dB for 100 turns on a 60 mmdiameter mandrel

[0121] Microbending sensitivity=3.0 (dB/km)/(g/mm) as determined by theexpandable bobbin method.

EXAMPLE 5

[0122]FIG. 11 illustrates another refractive-index profile of a fiberaccording to the second embodiment. Inner core 72 of fiber 10 has anα-profile shape with a α=4 and a maximum refractive-index difference Δn₁of about 0.0070. Inner core 72 extends for a radius r₁ of about 3.7 μm.The refractive-index difference of inner core 72 may be increased bydoping the width of the inner core with GeO₂ or any other well-knownrefractive-index-increasing dopant.

[0123] First glass layer 74 has a substantially parabolic depressedprofile and reaches a minimum refractive-index difference Δn₂ of about−0.0024 at a midpoint within its width of about 2.4 μm. Therefractive-index difference of first glass layer 74 may be decreased bydoping the width of the first core layer with fluorine or any otherwell-known refractive-index-decreasing dopant. Second glass layer 76 hasa refractive-index difference Δn₃ of about 0 and extends for a radialdistance of about 2.6 μm.

[0124] Third glass layer 78 has a substantially parabolic profile andreaches a maximum refractive index Δn₄ of about 0.0063 at a midpointwithin its width of about 2.1 μm. The refractive-index difference of thethird glass layer may be formed by doping the glass layer withincreasing amounts of GeO₂, or any other well-knownrefractive-index-increasing dopant.

[0125] Third glass layer 78 is surrounded by a depressed fourth glasslayer 79 that has a minimum refractive-index difference of about −0.001across its width of 2.9 μm.

[0126] Fourth glass layer 79 is surrounded by cladding 80 that has arefractive-index difference of about 0.

[0127] The specific embodiment of fiber 10 illustrated in FIG. 11 hasthe following optical transmission characteristics.

[0128] Cable cut off ≦1500 nm

[0129] Dispersion at 1550 nm=3.5 ps/nm/km

[0130] Dispersion Slope at 1550 nm=0.043 ps/nm²/km

[0131] Mode Field Diameter at 1550 nm=8.9 μm

[0132] Effective Area at 1550 nm=61 μm²

[0133] Non Linearity Coefficient γ=1.4 W⁻¹ km⁻¹

[0134] Macrobending attenuation <0.5 dB for 100 turns on a 60 mmdiameter mandrel

[0135] Microbending sensitivity=3.9 (dB/km)/(g/mm) as determined by theexpandable bobbin method.

[0136]FIG. 12 shows the relation between dispersion slope and depressedprofile volume for a set of fibers with refractive index profilesaccording to the conventional design of FIG. 5. Random sets of parametervalues defining refractive-index profiles according to FIG. 5 have beenchosen. The optical transmission characteristics of each set have beenevaluated by computer simulation.

[0137] Each cross in FIG. 12 represents a set of parameter valuescorresponding to a refractive index profile achieving opticaltransmission characteristics in the following ranges: Theoreticalcut-off <1800 nm Dispersion at 1550 nm 2-8 ps/nm/km Effective area 53-57μm²

[0138] Macrobending attenuation <0.5 dB for 100 turns on 60 mm diametermandrel

[0139] Microbending sensitivity <5 (dB/km)/(g/mm) by the expandablebobbin test method,

[0140] while parameter sets giving refractive-index profiles havingoptical transmission characteristics outside the above ranges have notbeen represented on FIG. 12.

[0141] A comparative example for a set of invention refractive-indexprofiles according to FIG. 4 is shown in FIG. 13. The selection criteriaare the same as above.

[0142] On both graphs a line 90 has been drawn corresponding to therelationship

S=0.07+V  (2)

[0143] between the dispersion slope S (in units of ps/nm²/km) and thedepressed profile volume V (in units of μm²).

[0144] As it is shown by above exemplary embodiments, there is a tradeoff between low dispersion slope and depressed profile volume. Forcorresponding fiber performances, the lower the dispersion slope, thehigher the depressed profile volume, thus increasing manufacturingcomplexity and dopant content.

[0145] The above graphs in FIGS. 12 and 13 show, however, thatrefractive index profiles corresponding to the conventional design ofFIG. 5 tend to have dispersion slope values greater than those given by(2), while refractive index profiles according to the present inventionhave dispersion slope values that are concentrated below those given by(2).

[0146] A refractive index profile of a fiber preform made by Applicantwith the MCVD technique is shown in FIG. 14. The preform comprises innercore region 72, first glass layer 74, second glass layer 76, third glasslayer 78, depressed fourth glass layer 79 and cladding 80.

[0147] The preform layers correspond to those of the invention fiberaccording to the embodiment described with reference to FIG. 3.

[0148] During the drawing process, in particular due to dopantdiffusion, the refractive-index profile for the drawn fiber may ingeneral undergo some changes from the preform refractive-index profile.In particular, second glass layer 76 is doped so as to achieve arefractive index difference value that is more negative than the desiredrefractive index value of the drawn fiber, to account for acorresponding small increase in the refractive index of this layerduring the drawing process. The amount of this increase can bedetermined by the skilled in the art, based on the preformcharacteristics and on the fiber drawing process parameters.

[0149] Applicant has determined that a preform having a refractive-indexprofile corresponding to that described in general with reference toFIG. 3, wherein in particular a second glass layer 76 has a refractiveindex difference that, in absolute value, is less than 40% of therefractive-index difference of a first glass layer 74, can be drawn by aconventional drawing process to give an optical fiber having desirableoptical transmission characteristics over the wavelength range 1450-1650nm, and having in particular an attenuation of about 0.21 dB/km at 1550nm, low macrobending and microbending losses and an improvedrelationship between dispersion slope and depressed profile volume.

[0150] Although the MCVD technique has been used to produce the preformshown in FIG. 14, other available vapor deposition techniques can beselected by the skilled in the art to the same end.

[0151] It will be apparent to one skilled in the art that variousmodifications and variations may be made to the fiber of the presentinvention without departing from the scope of the invention. Forexample, the refractive-index profiles depicted in the figures areintended to be exemplary of preferred embodiments. The precise shape,radial distance, and refractive-index differences may readily befluctuated by one of ordinary skill in the art to obtain equivalentfibers to those disclosed herein without departing from the scope ofthis invention. The present invention covers the modifications andvariations of this invention provided they come within the scope of theappended claims and their equivalents.

What is claimed is:
 1. A single-mode optical transmission fiber for usein a wavelength-division-multiplexing transmission system having carrierwavelengths in an extended wavelength range, the fiber comprising: aglass core including: an inner core having a first refractive-indexdifference; a first layer radially surrounding the inner core along thelength of the fiber and having a second refractive-index difference ofless than zero, a second layer radially surrounding the first layeralong the length of the fiber and having a third refractive-indexdifference, a third layer radially surrounding the second layer alongthe length of the fiber and having a fourth refractive-index differenceof greater than zero; and a glass cladding surrounding the glass coreand having a refractive-index difference substantially equal to zero,wherein the fiber has a dispersion slope less than about 0.07 ps/nm²/kmover the extended wavelength range, characterized in that said thirdrefractive-index difference is, in absolute value, less than 40% of saidsecond refractive-index difference,
 2. The fiber of claim 1, whereinsaid third refractive-index difference is, in absolute value, less than20% of said second refractive-index difference.
 3. The fiber of claim 2,wherein said third refractive-index difference is substantially zero. 4.The fiber according to at least one of claims 1-3, wherein said secondlayer has a width in the range 1-5 μm.
 5. The fiber of claim 4, whereinsaid second layer has a width in the range 2-4 μm.
 6. The fiberaccording to at least one of claims 1-5, further comprising a fourthlayer radially surrounding the third layer along the length of the fiberand having a fifth refractive-index difference of less than zero.
 7. Thefiber according to at least one of claims 1-6, wherein the firstrefractive-index difference of the inner core exceeds the fourthrefractive-index difference of the third layer.
 8. The fiber accordingto at least one of claims 1-6, wherein the fourth refractive-indexdifference of the third layer exceeds the first refractive-indexdifference of the inner core.
 9. The fiber according to at least one ofclaims 1-8, wherein the fiber has a dispersion slope less than about0.05 ps/nm²/km over the extended wavelength range.
 10. The fiberaccording to at least one of claims 1-9, wherein the fiber has adispersion value of at least 1.5 ps/nm/km over the extended wavelengthrange.
 11. The fiber of claim 10, wherein the dispersion value rangesfrom about 1.5-12 ps/nm/km across the extended wavelength range.
 12. Thefiber according to at least one of claims 1-11, wherein the extendedwavelength range is between about 1530 and 1650 nm.
 13. The fiber ofclaim 12, wherein the fiber has a dispersion slope less than or equal to0.043 ps/nm²/km at a wavelength of 1550 nm.
 14. The fiber according toat least one of claims 12-13, wherein the fiber has a zero-dispersionwavelength of less than about 1500 nm.
 15. The fiber of claim 14,wherein the fiber has a zero-dispersion wavelength of less than about1480 nm.
 16. The fiber according to at least one of claims 1-11, whereinthe extended wavelength range is between about 1450 and 1650 nm.
 17. Thefiber of claim 16, wherein the fiber has a dispersion slope less than orequal to 0.046 ps/nm²/km at a wavelength of 1550 nm.
 18. The fiberaccording to at least one of claims 16-17, wherein the fiber has azero-dispersion wavelength of less than about 1450 nm.
 19. The fiberaccording to at least one of claims 1-18, wherein the fiber has aneffective area of greater than 50 μm².
 20. The fiber according to claim19, wherein the fiber has an effective area of about 55 μm².
 21. Asingle-mode optical transmission fiber, comprising: a glass core havinga central cross-sectional area with a first refractive-index peak, anoutside ring with a second refractive-index peak higher than the firstpeak, a first intermediate region between the two peaks having alow-dopant content, and a second intermediate region between the firstpeak and the first intermediate region with a refractive-indexdepression lower than the first intermediate region; and a glasscladding surrounding the glass core, wherein the fiber has a dispersionslope of less than about 0.05 ps/nm²/km over a wavelength range of about1530-1650 nm.
 22. The fiber of claim 21, further comprising a layerradially surrounding the outside ring and having a depressedrefractive-index difference.
 23. The fiber according to at least one ofclaims 21-22 wherein the fiber has a dispersion value of at least 1.5ps/nm/km over a wavelength range of about 1530-1650 nm.
 24. The fiber ofclaim 23 wherein the fiber has a zero-dispersion wavelength of less than1500 nm.
 25. The fiber of claim 24 wherein the fiber has azero-dispersion wavelength of less than about 1480 nm.
 26. The fiberaccording to at least one of claims 21-22 wherein the fiber has adispersion slope of less than about 0.05 ps/nm²/km over a wavelengthrange of about 1450-1650 nm.
 27. The fiber of claim 26, wherein thefiber has a zero-dispersion wavelength of less than about 1450 nm. 28.The fiber according to at least one of claims 21-27, wherein the fiberhas an effective area of greater than 50 μm².
 29. The fiber according toclaim 28 wherein the fiber has an effective area of about 55 μm².
 30. Amethod for producing a single-mode optical fiber for use in awavelength-division-multiplexing transmission system having carrierwavelengths in an extended wavelength range, comprising: producing apreform having an inner core region with a first refractive-indexdifference; a first layer radially surrounding the inner core regionalong the length of the preform and having a second refractive-indexdifference of less than zero, a second layer radially surrounding thefirst layer along the length of the preform and having a thirdrefractive-index difference, a third layer radially surrounding thesecond layer along the length of the preform and having a fourthrefractive-index difference of greater than zero; and a glass claddinglayer surrounding the core region and having a refractive-indexdifference substantially equal to zero; and drawing said preform,characterized in that the step of producing a preform comprises:selecting said third refractive-index difference to be, in absolutevalue, less than 40% of said second refractive-index difference;selecting a width of said second layer in the preform so that acorresponding layer in the drawn fiber has a width in the range 1-5 μm.31. The method of claim 30, wherein said third refractive-indexdifference is selected to be, in absolute value, less than 20% of saidsecond refractive-index difference.
 32. The method according to at leastone of claims 30-31, wherein the step of producing a preform comprisesselecting a width of said second layer in the preform so that acorresponding layer in the drawn fiber has a width in the range 2-4 μm.33. The method according to at least one of claims 30-32, comprisingselecting the widths of said inner core region and of said first, secondand third layer and selecting said first, second, third and fourthrefractive index differences so that the dispersion slope of the drawnfiber is less than or equal to 0.046 ps/nm²/km at a wavelength of 1550nm.
 34. The method of claim 33, comprising selecting the widths of saidinner core region and of said first, second and third layer andselecting said first, second, third and fourth refractive indexdifferences so that the dispersion slope of the drawn fiber is less thanor equal to 0.043 ps/nm²/km at a wavelength of 1550 nm.